The 'subjective reduction' interpretation of measurement in quantum physics proposes that the collapse of the wave-packet, associated with measurement, is due to the consciousness of human observers. A refined conceptual replication of an earlier experiment, designed and carried out to test this interpretation in the 1970s, is reported. Two improvements are introduced. First, the delay between pre-observation and final observation of the same quantum event is increased from a few microseconds in the original experiment to one second in this replication. (...) Second, rather than using the final observers' verbal response as the dependent variable, his early brain responses as measured by EEG are used. These early responses cover a period during which an observer is not yet conscious of an observed event. Our results support the 'subjective reduction' hypothesis insofar as significant differences in the brain responses of the final observer are found, depending on whether or not the pre-observer has been looking at the quantum event . Alternative 'normal' explanations are discussed and rejected. It is concluded that the present results do justify further research along these lines. (shrink)